jbl vs css vs mb quart

Keep in mind that those people at the shop are used to selling to kids that think the bass being loud as possible is all that matters.  They don't realize that there are people that prefer SQ over SPL.  They may not be happy with the output of the MB Quart subs, but running two of them in a ported enclosure with that much power should be more than enough output for anyone.  And you are absolutely right about the 15" subs not necessarily being sloppy.  Unfortunately they were probably fed a line about how the heavier cone causes that.  But in actuality a properly designed 15 can sound just as good as any 10" sub.  Here's a great tech paper that disproves it: http://www.adireaudio.com/Files/WooferSpeed.pdf

I'd say since you aready know you like the JBL subs, that might be a good choice.  Is there a local dealer you can go listen to them again to be sure your tastes haven't changed over the years? 

Unfortunately it gets more complicated than that.  The Le rating is generally measured at 1 kHz.  The actual inductance down in the subwoofers pass band will normally be much higher.  Also the adition of copper in the pole piece will effect the Le, like the CSS sub uses as well as the TC sub mentioned earlier.  However this addition alone doesn't necessarily mean it will have a positive effect.  Dr. David Hyre of Adire Audio (who has his doctorate in Nuclear Magneto Resonance Spectroscopy) did quite a bit of research with adding copper in different locations within the motor structure and found there were certain point where it works best, and other points where it had no effect or even a negative effect on the inductance where it mattered.

Also, generally the subwoofers within the same line will have identical inductance (or really close to it) as they usually share the same motor parts.  Comparing inductance is helpful when comparing different subwoofers.  Also of note is that the JBL GTI sub with it's dual coils wired in reverse will have almost no inductance.

I'd be very leary of putting that much power on them.  I suppose you could just run one of your amps on a pair of them.  Your electrical system will probably thank you for it.